An electric spark discharge can produce an ignition in a combustion chamber of a combustion engine. The electric spark discharge across two spark plug electrodes creates a plasma within the combustion chamber. The plasma can ignite a fuel-air mixture that is present in the combustion chamber. This ignition technique is commonly referred to as spark ignition.
Spark ignition suffers from several drawbacks. First of all, there is relatively poor energy transfer efficiency. A relatively large proportion of electrical energy does not contribute to heating the plasma once this is formed. Moreover, the more electrical energy is used for spark ignition, the less efficient this energy is converted into heat. A further drawback is that the plasma that is formed is of relatively small volume, mainly located between two electrodes. The electrodes need to be relatively close to each other. If not, an excessively high voltage would be required for an electric spark discharge.
The drawbacks of spark ignition become particularly manifest in recently developed combustion concepts, which impose more difficult ignition conditions. Examples of such combustion concepts include those referred to as high dilution, high compression ratio, variable valve actuation, lean burn, gasoline direct injection in lean burn conditions, homogeneous charge compression ignition, and camless. The latter term refers to an engine without a camshaft, in which an electromagnetic or hydraulic actuator functionally replaces the camshaft. In one or more of these processes, ignition conditions may be difficult due to, for example, poor homogeneity of the fuel-air mixture, this mixture being highly diluted, and this mixture having a relatively high pressure when ignition should occur. By the way, the fuel-air mixture may contain a significant proportion of recirculated exhaust gas.
Spark ignition sets limits on achieving higher engine efficiency and lower emissions, which combustion processes such as those mentioned hereinbefore can provide. A main reason for this is that higher energy efficiency and lower emissions require an ignition with plasma that is relatively hot and voluminous. It is difficult, or even impossible, to create such plasma with spark ignition, in particular in a reliable and economical manner. For example, more electrical energy could be applied to a sparkplug to create hotter and more voluminous plasma. However, this would generally be at the expense of more erosion. The sparkplug would have a shorter lifetime.